UV-emitting phosphor blend and tanning lamp containing same

ABSTRACT

A UV-emitting phosphor blend is provided which comprises a mixture of at least two UVA-emitting phosphors selected from SrB 4 O 7 :Eu, YPO 4 :Ce, and BaSi 2 O 5 :Pb wherein the sum of the weight percentages of the phosphors in the mixture equals 100%. The phosphor blend may additionally contain an amount of SrCe 0.08 MgAl 11 O 18  in an amount from 0 to 12 wt. % of the mixture. Tanning lamps containing these blends exhibit increases in initial UVA output and UVA maintenance.

TECHNICAL FIELD

This invention relates to ultraviolet-emitting phosphors and phosphor blends. More, particularly this invention relates to the UVA output and maintenance of tanning lamps.

BACKGROUND OF THE INVENTION

Tanning lamps are similar to conventional mercury-vapor fluorescent lamps in construction except that they contain one or more phosphors which emit radiation in the ultraviolet (UV) region of the electromagnetic spectrum in order to induce a tanning response in human skin. The UV emission from tanning lamps is classified into two regions, UVA and UVB. The U.S. Food & Drug Administration (FDA) defines the UVA region as being from 320–400 nm and the UVB region as being from 260–320 nm. (See, Performance Standards for Light-Emitting Products: Sunlamp Products and Ultraviolet Lamps intended for use in Sunlamp Products, 21 C.F.R. 1040.20) The majority of the UV flux of tanning lamps is emitted in the UVA region with the balance in the UVB region. This is intended to generally mimic the relative proportions of UVA and UVB in natural sunlight. Both UVA and UVB have a role in the tanning process. When human skin is exposed to UV light, the melanocytes in the skin's epidermis release extra melanin, a protein pigment responsible for tanning. While UVB radiation is more efficient than UVA in stimulating the production of melanin, UVA radiation is much more efficient than UVB in oxidizing the melanin to produce the brown color which is characteristic of suntans.

The typical UV-emitting phosphor employed in tanning lamps is a single-component lead-activated barium disilicate, BaSi₂O₅:Pb. The BaSi₂O₅:Pb phosphor intrinsically has low maintenance due to its affinity for mercury. Hence, when this phosphor is used in a mercury-vapor fluorescent lamp, the phosphor surface is usually protected by a coating, typically aluminum oxide, as described in U.S. Pat. Nos. 4,585,673, 4,710,674, 4,825,124 and 5,523,018. The alumina coating provides a barrier layer between the phosphor and the mercury-vapor and improves long term-UV maintenance.

Reflector tanning lamps (RTLs) are a special class of tanning lamps wherein the UV flux is focused by a reflector which causes the flux to be emitted over a narrower segment of the circumference, for example 140°, instead of through the entire 360° circumference of the lamp. Two key performance parameters of tanning lamps, and RTLs in particular, are the initial UVA output of the lamp and the UVA maintenance. The initial UVA output of the lamp (0 hours) will be herein referred to as the 0 h UVA output. The UVA maintenance measures the ability of the lamp to maintain its initial UVA output over a period of time, usually the first 100 hours of operation, and is expressed as a percentage of the initial output. As used herein, the 100 h UVA maintenance is defined as the ratio of the UVA output of the lamp after 100 hours of operation to its initial UVA output times 100% ((100 h UVA output/0 UVA output)×100%). For commercial applications, it is very desirable to have both a high 0 h UVA output and a high 100 h UVA maintenance. Unfortunately, current commercial RTLs either have a high 0 h UVA output (≧8600 μW/cm²) or a high 100 h UVA maintenance (≧87%) but not both attributes simultaneously. In other words, RTLs that have a high 100 h UVA maintenance (≧87%) have a 0 h UVA output that is much less than 8600 μW/cm². Conversely, RTLs that have a high UVA output (≧8600 μW/cm²) have a 100 h UVA maintenance that is much less than 87%. The inverse relationship between 0 h UVA output and 100 h UVA maintenance for commercial reflector tanning lamps is shown in FIG. 1.

Therefore, it would be an advantage over the present state of the art to have tanning lamps, and RTLs in particular, that exhibit higher 0 h UVA outputs at high 100 h UVA maintenances.

SUMMARY OF THE INVENTION

It is an object of the invention to obviate the disadvantages of the prior art.

It is another object of the invention to provide tanning lamps having higher 0 h UVA outputs at high 100 h UVA maintenances.

In accordance with one aspect of the invention, there is provided a UV-emitting phosphor blend which comprises a mixture of at least two UVA-emitting phosphors selected from SrB₄O₇:Eu, YPO₄:Ce, and BaSi₂O₅:Pb, wherein the sum of the weight percentages of the phosphors in the mixture equals 100%. The phosphor blend additionally contains an optional amount of SrCe_(0.08)MgAl₁₁O₁₈ having a range from 0 to 12 wt. % of the mixture.

In accordance with another aspect of the invention, there is provided a fluorescent tanning lamp having a UV-transmissive envelope and a phosphor coating on an inner wall of the envelope. The phosphor coating contains a UV-emitting phosphor blend comprising a mixture of at least two UVA-emitting phosphors selected from SrB₄O₇:Eu, YPO₄:Ce, and BaSi₂O₅:Pb. The phosphor blend additionally contains an optional amount of SrCe_(0.08)MgAl₁₁O₁₈ having a range from 0 to 12 wt. % of the mixture.

In accordance with a further aspect of the invention, there is provided a fluorescent tanning lamp having a tubular UV-transmissive envelope, a reflector layer, and a UV-emitting phosphor layer. The reflector layer extends over a 220° segment of the circumference of the envelope and is disposed between the envelope and the phosphor layer. The phosphor layer extends over the entire circumference of the envelope and the lamp exhibits a 0 h UVA output of at least about 8800 μW/cm² and a 100 h UVA maintenance of at least about 87%.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph of the relationship between the 0 h UVA output and the 100 h UVA maintenance of commercial reflector tanning lamps.

FIG. 2 is a contour plot of the 0 h UVA output of several groups of tanning lamps containing various UV-emitting phosphor blends.

FIG. 3 is a contour plot of the 100 h UVA maintenance of several groups of tanning lamps containing various UV-emitting phosphor blends.

FIG. 4 is a graphical illustration of the preferred UV-emitting blend compositions of the present invention.

FIG. 5 is a cross-sectional illustration of a reflector tanning lamp.

DETAILED DESCRIPTION OF THE INVENTION

For a better understanding of the present invention, together with other and further objects, advantages and capabilities thereof, reference is made to the following disclosure and appended claims taken in conjunction with the above-described drawings.

We have discovered that it is possible to simultaneously achieve a high 0 h UVA output at a high 100 h UVA maintenance by using a blend containing a mixture of at least two UV-emitting phosphors selected from the group comprising SrB₄O₇:Eu, YPO₄:Ce, and BaSi₂O₅:Pb. All three phosphors in the group emit radiation in the UVA region. The first phosphor, SrB₄O₇:Eu, exhibits an emission spectrum which peaks at 368 nm. The second phosphor, YPO₄:Ce, exhibits a bimodal emission spectrum with local peaks at 338 and 356 nm. The third phosphor, BaSi₂O₅:Pb, has an emission spectrum that peaks at 351 nm. As described previously, the BaSi₂O₅:Pb phosphor is the phosphor of choice in current state-of-the-art RTL applications. For example, all of the lamps shown in FIG. 1 use this phosphor at 100%. Although not required for this invention, it is preferred that an alumina coating be applied to the BaSi₂O₅:Pb phosphor.

The UV-emitting blend may additionally contain an amount of SrCe_(0.08)MgAl₁₁O₁₈, a phosphor which emits in the UVB region at 310.5 nm. The amount of this phosphor may range from 0–12 weight percent (wt. %) based on the combined weight of the UVA-emitting phosphors in the blend.

In a first alternate embodiment, the UV-emitting phosphor blend of this invention comprises a mixture of BaSi₂O₅:Pb and at least one phosphor selected from SrB₄O₇:Eu and YPO₄:Ce. The relative proportions of the phosphors in the mixture range from 0 to 50 wt. % SrB₄O₇:Eu, 0 to 40 wt. % YPO₄:Ce, and 30 to 70 wt. % BaSi₂O₅:Pb wherein the sum of the weight percentages equals 100%. Preferably, the relative proportions of the UVA-emitting phosphors in the mixture range from 30 to 40 wt. % SrB₄O₇:Eu, 30 to 40 wt. % YPO₄:Ce, and 30 to 40 wt. % BaSi₂O₅:Pb. More preferably, the proportions range from 30 to 35 wt. % SrB₄O₇:Eu, 30 to 35 wt. % YPO₄:Ce, and 30 to 35 wt. % BaSi₂O₅:Pb. And most preferably, the proportions of the mixture are about 33.3 wt. % SrB₄O₇:Eu, about 33.3 wt. % YPO₄:Ce, and about 33.3 wt. % BaSi₂O₅:Pb. In each of the above cases, the UV-emitting phosphor blend may additionally contain an optional amount of the UVB-emitting SrCe_(0.08)MgAl₁₁O₁₈ phosphor in an amount from 0 to 12 wt. % of the mixture of UVA-emitting phosphors.

In a second alternate embodiment, the UV-emitting phosphor blend comprises a mixture of BaSi₂O₅:Pb and at least one phosphor selected from SrB₄O₇:Eu and YPO₄:Ce, wherein the relative proportions of the mixture range from 0 to 20 wt. % SrB₄O₇:Eu, 0 to 20 wt. % YPO₄:Ce, and 50 to 70 wt. % BaSi₂O₅:Pb and the sum of the weight percentages of the phosphors in the mixture equals 100%. Preferably, the relative proportions of the UVA-emitting phosphors range from 15 to 20 wt. % SrB₄O₇:Eu, 15 to 20 wt. % YPO₄:Ce, and 65 to 70 wt. % BaSi₂O₅:Pb. More preferably, the proportions are about 16.7 wt. % SrB₄O₇:Eu, about 16.7 wt. % YPO₄:Ce, and about 66.6 wt. % BaSi₂O₅:Pb. Again, in each instance, the UV-emitting phosphor blend may additionally contain SrCe_(0.08)MgAl₁₁O₁₈ in an amount from 0 to 12 wt. % of the mixture of UVA-emitting phosphors.

In a third alternate embodiment, the UV-emitting phosphor blend comprises a mixture of BaSi₂O₅:Pb and SrB₄O₇:Eu, wherein the relative proportions range from 40 to 60 wt. % SrB₄O₇:Eu and 40 to 60 wt. % BaSi₂O₅:Pb and the sum of their weight percentages equals 100%. Preferably, the relative proportions range from 45 to 55 wt. % SrB₄O₇:Eu and 45 to 55 wt. % BaSi₂O₅:Pb. More preferably, the mixture comprises about 50% wt. % SrB₄O₇:Eu and about 50 wt. % BaSi₂O₅:Pb. As above, the phosphor blend may additionally contain SrCe_(0.08)MgAl₁₁O₁₈ in an amount from 0 to 12 wt. % of the mixture of UVA-emitting phosphors.

In a fourth alternate embodiment, the UV-emitting phosphor blend comprises a mixture of YPO₄:Ce and SrB₄O₇:Eu, wherein the relative proportions of the phosphors range from 40 to 60 wt. % SrB₄O₇:Eu and 40 to 60 wt. % YPO₄:Ce and the sum of their weight percentages equals 100%. Preferably, this mixture comprises 45 to 55 wt. % SrB₄O₇:Eu and 45 to 55 wt. % YPO₄:Ce. More preferably, the mixture comprises about 50% wt. % SrB₄O₇:Eu and about 50 wt. % YPO₄:Ce. As above, the phosphor blend may additionally contain SrCe_(0.08)MgAl₁₁O₁₈ in an amount from 0 to 12 wt. % of the mixture of UVA-emitting phosphors.

The following nonlimiting examples are presented.

EXAMPLES

A number of reflector tanning lamps were made which contained various combinations of the four UV-emitting phosphors used in the UV-emitting blends of this invention. All the four UV-emitting phosphors were manufactured by OSRAM SYLVANIA Products Inc., Towanda, Pa. SrB₄O₇:Eu is designated as OSRAM SYLVANIA Type 2052, BaSi₂O₅:Pb (coated with Al₂O₃) as OSRAM SYLVANIA Type GS2016, YPO₄:Ce as OSRAM SYLVANIA Type 2040, and SrCe_(0.08)MgAl₁₁O₁₈ as OSRAM SYLVANIA Type 2096. The physical properties of the phosphors are given in Table 1. Particle size distribution was measured by a Coulter LS130 laser diffraction system.

TABLE 1 BET Surface Phosphor 50% Size (μm) Area (m²/g) Density (g/cm³) SrB₄O₇:Eu 9.9 0.61 3.4 YPO₄:Ce 13.5 0.69 4.24 BaSi₂O₅:Pb 11.2 0.65 3.9 SrCe_(0.08)MgAl₁₁O₁₈ 12.9 0.91 3.5

The effect of the blend composition on the 0 h UVA output and 100 h UVA maintenance of the lamps may be demonstrated with a Simplex-Centroid design. In particular, the blend composition was varied such that the sum of the mass fractions of the three UVA-emitting phosphors, SrB₄O₇:Eu, YPO₄:Ce and BaSi₂O₅:Pb, added up to unity. The Simplex-Centroid design for the three UVA-emitting phosphors results in a total of ten blends for which the relative amounts of each phosphor are given in Table 2. Each of these ten blends also contained a fixed amount of the UVB-emitting phosphor, SrCe_(0.08)MgAl₁₁O₁₈, which was 4 wt. % of the mixture of UVA-emitting phosphors. Blend 11 was used as a separate control and contained only the BaSi₂O₅:Pb phosphor.

TABLE 2 Blend/Group SrB₄O₇:Eu YPO₄:Ce BaSi₂O₅:Pb  1 1 0 0  2 0 1 0  3 0 0 1  4 0.5 0.5 0  5 0.5 0 0.5  6 0 0.5 0.5  7 0.3333 0.3333 0.3333  8 0.6667 0.1667 0.1667  9 0.1667 0.6667 0.1667 10 0.1667 0.1667 0.6667 11 0 0 1 (Control)

These blends are represented in the triangular composition diagrams shown in FIGS. 2–4. Blends 1–3 (Gr 1–3) occupy the vertices of the ternary diagrams. Blends 4 to 6 (Gr 4–6) occupy the mid-points of the three sides of the triangles. For example, blend 5 contains a 50:50 mixture of SrB₄O₇:Eu and BaSi₂O₅:Pb and is located exactly halfway between the vertices representing SrB₄O₇:Eu and BaSi₂O₅:Pb. Blends 7 to 10 (Gr 7–10) contain mixtures of SrB₄O₇:Eu, YPO₄:Ce and BaSi₂O₅:Pb. In particular, blend 7 is the centroid which is located at the point of intersection of the medians of the triangle. At the centroid, each of the three phosphors is a third of the mixture. Blends 8 to 10 are called axial points and are located on the median lines of the triangle such that one of the components is ⅔ of the mixture while the other two are a sixth each. It is important to remember that each of the ten blends contains 4 wt. % SrCe_(0.08)MgAl₁₁O₁₈ wherein the weight percentage is calculated based on the combined weight of the mixture of UVA-emitting phosphors.

Coating Suspensions and Bulb Coating

Organic-based coating suspensions were made using conventional suspension components. These included, in addition to the phosphor blend, a solvent, a polymeric binder, a dispersing agent, a plasticizer and adherence agents. A preferred suspension includes xylene as the solvent, ethyl cellulose as the binder, and benzoic acid and Armeen CD, a primary amine made by Akzo Nobel Chemicals, as the dispersing agent combination. In addition, dibutyl phthalate is a common plasticizer and finely divided aluminum oxide (Aluminum Oxide C from Degussa AG) serves as a good adherence agent.

Standard six-foot T12 (1.5 in. O. D.) bulbs with a precoated alumina reflector covering about 220° of the circumference were used. The phosphor suspension was manually applied over the interior surface using a combination of axial and rotational motions. The coated bulb was then allowed to drain and dry for about 30 minutes yielding a phosphor coating having a total loading of about 8.5 g of phosphor.

Lamp Finishing

The lamps were then loaded into a baker where the residual organic components in the coating were combusted in the presence of air. Electrode mounts were sealed into each end of the baked bulbs. The lamps were then preheated and exhausted through exhaust tubes in the electrode mounts. Mercury and a fill gas were introduced into the lamps and the exhaust tubes were sealed. The fill pressure was 1.8 torr for the ten test groups and 2.7 torr for the control group. The fill gas in all cases was a 70/30 neon-argon mixture. Mercury dosing was 18 mg. Conventional lamp bases were there attached to the lamps. The wattages of the test groups ranged from a low of 167.3 W to a high of 170.2 W whereas the control group wattage was 170.9 W.

A cross-sectional illustration of a finished reflector lamp is shown in FIG. 5. The lamp UV-transmissive envelope 3 has a partial alumina reflector coating 5 which extends over about 220° of the circumference. The phosphor coating 7 is disposed on top of the reflector coating 5 and extends over the entire circumference of the envelope 3 including the 140° segment 9 where there is no reflector coating.

Several lamps were made with each phosphor blend. The UV output of each group of test lamps was corrected for the difference in wattages between the test group and the control group. This was done by (1) computing the UV output per watt for each test group, (2) multiplying that value by the wattage differential between each test group and the control group 11, and (3) adding the result to the original UV flux value for the test group. A second correction factor was then applied to account for differences between the experimental measurement setup used for these lamps and a typical measurement setup used for commercial lamps. One of the reasons for the different experimental setup used here was that the UV output from some of lamps was too high to permit the measurement to be made at the normal 10 cm distance from the lamp centerline. A second reason was that a different lamp stabilization method was adopted for these tests that increased the signal-to-noise ratio of the photometric measurement.

After determining the initial UVA output, five lamps per group were aged to 100 h and then the UVA output was measured again. The 100 h UVA maintenance was then computed for each of the lamps in all groups. As described previously, the 100 h UVA maintenance is defined as the ratio of 100 h UVA output to 0 h UVA output with the ratio being multiplied by 100 to express it as a percentage. The 100 h maintenance of each group was then adjusted by normalizing the maintenance of the control group to 85% which is typical for this lamp. The mean 0 h UVA output and 100 h UVA maintenance of the groups is shown in Table 3.

TABLE 3 Corrected 0 h UVA Output Normalized 100 h UVA Group/Blend (μW/cm²) Maintenance (%)  1 7364 91.3  2 7816 87.1  3 8641 84.7  4 7887 92.3  5 9099 89.6  6 8790 84.5  7 8816 90.5  8 8409 89.7  9 8525 87.8 10 8896 87.2 11 8388 85.0 (Control) From the data in Table 3, it is possible to construct contour diagrams in the triangular composition space. Referring to FIG. 2, there is shown a contour plot for the 0 h UVA output. Each contour line connects those compositions which have the same 0 h UVA output. The 0 h UVA output associated with each contour line is given in μW/cm².

It is apparent that a higher 0 h UVA output is favored by compositions that lie closer to the SrB₄O₇:Eu—BaSi₂O₅:Pb binary line. The blends having the three highest 0 h UVA outputs in the Simplex-Centroid design were blends 5, 10 and 7 with 0 h UVA outputs of 9100, 8896 and 8816 μW/cm² respectively. These outputs exceed the 0 h UVA output of the control group 11 by 8.5%, 6%, and 5.1%, respectively. These increases are greater than the approximate 3% increase which is attributable to the lower fill pressure of the test lamps. The increase in the 0 h UVA output caused by the lower fill pressure alone can be seen by comparing the 0 h UVA outputs of groups 3 and 11.

Blend 5 contains no YPO₄:Ce and its 0 h UVA output greatly exceeds the mean 0 h UVA output of blends 1 and 3 which represent single-component mixtures of each phosphor in blend 5. This indicates that there is a synergism between the SrB₄O₇:Eu and BaSi₂O₅:Pb components which cannot be attributed to wattage or powder weight issues. While not wishing to be bound by theory, a possible explanation for the synergy between the SrB₄O₇:Eu and BaSi₂O₅:Pb phosphors may be deduced from the observed overlap between the emission spectrum of the BaSi₂O₅:Pb phosphor and the excitation spectrum of the SrB₄O₇:Eu phosphor. The excitation spectrum for SrB₄O₇:Eu extends up to about 360 nm and the broad emission from BaSi₂O₅:Pb peaks at about 350 nm. This means that a portion of the emission from the BaSi₂O₅:Pb phosphor excites the SrB₄O₇:Eu phosphor which then remits according to its characteristic emission spectrum which peaks at 368 nm. The overlap of the excitation and emission spectra coupled with quantum efficiency considerations of individual phosphor components may explain the significant synergy identified for the SrB₄O₇:Eu—BaSi₂O₅:Pb system.

A similar contour plot was constructed for the 100 h UVA maintenance. Referring to FIG. 3, the contour lines define the range of compositions having the same 100 h UVA maintenance value. The maintenance value associated with each contour line is given in terms of %. The plot shows that higher 100 h UVA maintenance values are favored by blend compositions that lie closer to the SrB₄O₇:Eu—YPO₄:Ce composition line. The highest 100 h UVA maintenance is observed with blend 4 which contains equal amounts of SrB₄O₇:Eu and YPO₄:Ce The plot shows that increasing the amount of BaSi₂O₅:Pb phosphor in the blend lowers the 100 h UVA maintenance.

From FIGS. 2 and 3, it is clear that the blend composition space that favors a high 0 h UVA output is different from the space that is optimal for high 100 h UVA maintenance. Therefore, there must be a joint optimization of the two responses. This was done with the help of an overlaid contour plot, FIG. 4, wherein the lower boundary for the 0 h UVA response was specified at about 8800 μW/cm² and lower boundary for the 100 h UVA maintenance response was specified at about 87%. The cross-hatched region in FIG. 4 is where the two responses overlap. This region represents those blend compositions that result in simultaneous high values of 0 h UVA output and 100 h UVA maintenance. It can be seen that the blends 5, 7 and 10 fall within this region.

The performance of these three blends can be put in perspective when they are compared with the current state of the art represented by control group 11. The lamps in the control group exhibit a 0 h UVA output of 8388 μW/cm² and a 100 h UVA maintenance of 85%. The lamps in group 5 exhibit a 0 h UVA output of 9099 μW/cm² and a 100 h UVA maintenance of 89.6%. This represents a gain of 8.5% in 0 h UVA output and an increase of almost 5 percentage points in 100 h UVA maintenance. Similarly, the lamps in groups 7 and 10 represent a gain of about 5–6% in 0 h UVA output and an increase of about 2 to more than 4 percentage points in 100 h UVA maintenance. As discussed previously, these gains in 0 h output are greater than the 3% increase attributable to the lower fill pressure of the test lamps. Thus, the UV-emitting blends of this invention have been shown to exhibit higher 0 h UVA outputs at high 100 h UVA maintenance values.

It is also worth noting at this point that blend 4 which has a lower 0 h UVA output than the control group is nevertheless commercially important because of its very high 100 h UVA maintenance. Even though the 0 h UVA output of blend 4 is 7887 μW/cm2 compared to 8388 μW/cm² for the control, the much higher 100 h UVA maintenance of 92.3% for blend 4 means that at 100 hours the UVA output of blend 4 (7280 μW/cm²) exceeds that of the control (7130 μW/cm²). Therefore, blend 4 would be very desirable for applications in which a very high maintenance is needed.

While there has been shown and described what are at the present considered the preferred embodiments of the invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the scope of the invention as defined by the appended claims. 

1. A UV-emitting phosphor blend comprising a mixture of at least two UVA-emitting phosphors selected from SrB₄O7:Eu, YPO₄:Ce, and BaSi₂O₅:Pb, the mixture of UVA-emitting phosphors comprising 0 to 50 wt. % SrB₄O₇:Eu, 0 to 40 wt. % YPO₄:Ce, and 30 to 70 wt. % BaSi₂O₅:Pb wherein the sum of the weight percentages of the phosphors in the mixture equals 100%, the phosphor blend additionally containing an optional amount of SrCe_(0.08)MgAl₁₁O₁₈ having a range from 0 to 12 wt. % of the mixture.
 2. The phosphor blend of claim 1 wherein the mixture of UVA-emitting phosphors comprises 30 to 40 wt. % SrB₄O₇:Eu, 30 to 40 wt. % YPO₄:Ce, and 30 to 40 wt. % BaSi₂O₅:Pb.
 3. The phosphor blend of claim 1 wherein the mixture of UVA-emitting phosphors comprises 30 to 35 wt. % SrB₄O₇:Eu, 30 to 35 wt. % YPO₄:Ce, and 30 to 35 wt. % BaSi₂O₅:Pb.
 4. The phosphor blend of claim 1 wherein the mixture of UVA-emitting phosphors comprises about 33.3 wt. % SrB₄O₇:Eu, about 33.3 wt. % YPO₄:Ce, and about 33.3 wt. % BaSi₂O₅:Pb.
 5. The phosphor blend of claim 1 wherein the mixture of UVA-emitting phosphors comprises 0 to 20 wt. % SrB₄O₇:Eu, 0 to 20 wt. % YPO₄:Ce, and 50 to 70 wt. % BaSi₂O₅:Pb.
 6. The phosphor blend of claim 1 wherein the mixture of UVA-emitting phosphors comprises 15 to 20 wt. % SrB₄O₇:Eu, 15 to 20 wt. % YPO₄:Ce, and 65 to 70 wt. % BaSi₂O₅:Pb.
 7. The phosphor blend of claim 1 wherein the mixture of UVA-emitting phosphors comprises about 16.7 wt. % SrB₄O₇:Eu, about 16.7 wt. % YPO₄:Ce, and about 66.6 wt. % BaSi₂O₅:Pb.
 8. A UV-emitting phosphor blend comprising a mixture of 45 to 55 wt. % SrB₄O₇:Eu and 45 to 55 wt. % BaSi₂O₅:Pb wherein the sum of the weight percentages of the phosphors in the mixture equals 100%, the phosphor blend additionally containing an optional amount of SrCe_(0.08)MgAl₁₁O₁₈ having a range from 0 to 12 wt. % of the mixture.
 9. The phosphor blend of claim 8 wherein the mixture of UVA-emitting phosphors comprises about 50 wt. % SrB₄O₇:Eu and about 50 wt. % BaSi₂O₅:Pb.
 10. A UV-emitting phosphor blend comprising a mixture of 40 to 60 wt. % SrB₄O₇:Eu and 40 to 60 wt. % YPO₄:Ce wherein the sum of the weight percentages of the phosphors in the mixture equals 100%, the phosphor blend additionally containing an optional amount of SrCe_(0.08)MgAl₁₁O₁₈ having a range from 0 to 12 wt. % of the mixture.
 11. The phosphor blend of claim 10 wherein the mixture of UVA-emitting phosphors comprises 45 to 55 wt. % SrB₄O₇:Eu and 45 to 55 wt. % YPO₄:Ce.
 12. The phosphor blend of claim 10 wherein the mixture of UVA-emitting phosphors comprises about 50 wt. % SrB₄O₇:Eu and about 50 wt. % YPO₄:Ce.
 13. A fluorescent tanning lamp having a UV-transmissive envelope and a phosphor coating on an inner wall of the envelope, the phosphor coating containing a UV-emitting phosphor blend comprising a mixture of at least two UVA-emitting phosphors selected from SrB₄O₇Eu, YPO₄:Ce, and BaSi₂O₅:Pb, the phosphor blend additionally containing an optional amount of SrCe_(0.08)MgAl₁₁O₁₈ having a range from 0 to 12 wt. % of the mixture, wherein the lamp exhibits a 100 h UVA maintenance of at least about 87%.
 14. The fluorescent lamp of claim 13 wherein the lamp exhibits a 0 h UVA output of at least about 8800 μW/cm₂.
 15. The fluorescent lamp of claim 14 wherein the mixture of UVA-emitting phosphors comprises 0 to 50 wt. % SrB₄O₇:Eu, 0 to 40 wt. % YPO₄:Ce, and 30 to 70 wt. % BaSi₂O₅:Pb.
 16. The fluorescent lamp of claim 14 wherein the mixture of UVA-emitting phosphors comprises 0 to 20 wt. % SrB₄O₇:Eu, 0 to 20 wt. % YPO₄:Ce, and 50 to 70 wt. % BaSi₂O₅:Pb.
 17. The fluorescent lamp of claim 14 wherein the mixture of UVA-emitting phosphors comprises 40 to 60 wt. % SrB₄O₇:Eu and 40 to 60 wt. % BaSi₂O₅:Pb.
 18. The fluorescent lamp of claim 14 wherein the envelope is tubular and further contains a reflector layer extending over a 220° segment of the circumference of the envelope, the reflector layer being disposed between the envelope and the phosphor layer, the phosphor layer extending over the entire circumference of the envelope.
 19. The fluorescent lamp of claim 13 wherein the mixture of UVA-emitting phosphors comprises 40 to 60 wt. % SrB₄O₇:Eu and 40 to 60 wt. % YPO₄:Ce.
 20. A fluorescent tanning lamp having a tubular UV-transmissive envelope, a reflector layer, and a UV-emitting phosphor layer, the reflector layer extending over a 220° segment of the circumference of the envelope and being disposed between the envelope and the phosphor layer, the phosphor layer extending over the entire circumference of the envelope, the lamp exhibiting a 0 h UVA output of at least about 8800 μW/cm² and a 100 h UVA maintenance of at least about 87%.
 21. A UV-emitting phosphor blend comprising a mixture of at least two UVA-emitting phosphors selected from SrB₄O₇:Eu, YPO₄:Ce, and BaSi₂O₅:Pb, the mixture of UVA-emitting phosphors comprising 0 to 50 wt. % SrB₄O₇:Eu, 0 to 40 wt. % YPO₄:Ce, and 30 to 70 wt. % BaSi₂O₅:Pb wherein the sum of the weight percentages of the phosphors in the mixture equals 100%.
 22. The phosphor blend of claim 21 wherein the mixture of UVA-emitting phosphors comprises 30 to 40 wt. % SrB₄O₇:Eu, 30 to 40 wt. % YPO₄:Ce, and 30 to 40 wt. % BaSi₂O₅:Pb.
 23. The phosphor blend of claim 21 wherein the mixture of UVA-emitting phosphors comprises 30 to 35 wt. % SrB₄O₇:Eu, 30 to 35 wt. % YPO₄:Ce, and 30 to 35 wt. % BaSi₂O₅:Pb.
 24. The phosphor blend of claim 21 wherein the mixture of UVA-emitting phosphors comprises about 33.3 wt. % SrB₄O₇:Eu, about 33.3 wt. % YPO₄:Ce, and about 33.3 wt. % BaSi₂O₅:Pb.
 25. The phosphor blend of claim 21 wherein the mixture of UVA-emitting phosphors comprises 0 to 20 wt. % SrB₄O₇:Eu, 0 to 20 wt. % YPO₄:Ce, and 50 to 70 wt. % BaSi₂O₅:Pb.
 26. The phosphor blend of claim 21 wherein the mixture of UVA-emitting phosphors comprises 15 to 20 wt. % SrB₄O₇:Eu, 15 to 20 wt. % YPO₄:Ce, and 65 to 70 wt. % BaSi₂O₅:Pb.
 27. The phosphor blend of claim 21 wherein the mixture of UVA-emitting phosphors comprises about 16.7 wt. % SrB₄O₇:Eu, about 16.7 wt. % YPO₄:Ce, and about 66.6 wt. % BaSi₂O₅:Pb.
 28. A UV-emitting phosphor blend comprising a mixture of 45 to 55 wt. % SrB₄O₇:Eu and 45 to 55 wt. % BaSi₂O₅:Pb wherein the sum of the weight percentages of the phosphors in the mixture equals 100%.
 29. The phosphor blend of claim 28 wherein the mixture of UVA-emitting phosphors comprises about 50 wt. % SrB₄O₇:Eu and about 50 wt. % BaSi₂O₅:Pb.
 30. A UV-emitting phosphor blend comprising a mixture of 40 to 60 wt. % SrB₄O₇:Eu and 40 to 60 wt. % YPO₄:Ce wherein the sum of the weight percentages of the phosphors in the mixture equals 100%.
 31. The phosphor blend of claim 30 wherein the mixture of UVA-emitting phosphors comprises 45 to 55 wt. % SrB₄O₇:Eu and 45 to 55 wt. % YPO₄:Ce.
 32. The phosphor blend of claim 30 wherein the mixture of UVA-emitting phosphors comprises about 50 wt. % SrB₄O₇:Eu and about 50 wt. % YPO₄:Ce.
 33. A fluorescent tanning lamp having a UV-transmissive envelope and a phosphor coating on an inner wall of the envelope, the phosphor coating containing a UV-emitting phosphor blend comprising a mixture of at least two UVA-emitting phosphors selected from SrB₄O₇:Ce, and YPO4:Ce, and BaSi₂O₅:Pb wherein the lamp exhibits a 100 h UVA maintenance of at least about 87%.
 34. The fluorescent lamp of claim 33 wherein the lamp exhibits a 0 h UVA output of at least about 8800 μW/cm².
 35. The fluorescent lamp of claim 34 wherein the mixture of UVA-emitting phosphors comprises 0 to 50 wt. % SrB₄O₇:Eu, 0 to 40 wt. % YPO₄:Ce, and 30 to 70 wt. % BaSi₂O₅:Pb.
 36. The fluorescent lamp of claim 38 wherein the mixture of UVA-emitting phosphors comprises 0 to 20 wt. % SrB₄O₇:Eu, 0 to 20 wt. % YPO₄:Ce, and 50 to 70 wt. % BaSi₂O₅:Pb.
 37. The fluorescent lamp of claim 34 wherein the mixture of UVA-emitting phosphors comprises 40 to 60 wt. % SrB₄O₇:Eu and 40 to 60 wt. % BaSi₂O₅:Pb.
 38. The fluorescent lamp of claim 33 wherein the mixture of UVA-emitting phosphors comprises 40 to 60 wt. % SrB₄O₇:Eu and 40 to 60 wt. % YPO₄:Ce. 